Commutator carbon brush and method of its manufacture



A ril 28, 1970 'P'. CONRATH 3,509,400

" COMMUTATOR CARBON BRUSH AND METHOD OF ITS MANUFACTURE Filed May 17,19s? 3,509,400 COMMUTATOR CARBON BRUSH AND METHOD OF ITS MANUFACTUREPaul Conrath, Bad Ischl, Austria, assignor to Sigri ElektrographitGesellschaft mit beschrankter Haftung, Meitingen uber Augsburg, Germany,a corporation of Germany Filed May 17, 1967, Ser. No. 639,221 Claimspriority, application Germany, May 17, 1966, S 103,855 Int. Cl. H02k5/14; H011 39/24 US. Cl. 310-248 2 Claims ABSTRACT OF THE DISCLOSURE Myinvention relates to commutator brushes of carbon, such as those made ofindustrial carbon or electrographite, and has for its principal objectto secure a high ratio of transverse resistance to longitudinalresistance.

A highest feasible transverse resistance of commutator brushes isdesirable for securing a satisfactory commutation performance,particularly in alternating-current commutator machines operating withhigh commutator lamination or sector voltages. Adverse to thisdesideratum is the slight anisotropy of electrographite brushes. As arule, such brushes exhibit a transverse to longitudinal resistance ratioof no more than 1.1:1 to 1.3:1. For example, an electrographite brush,on the average, has a longitudinal resistivity of about 40 ohm mmF/m.and a transverse resistivity of about 52 ohm mmF/m.

The requirement for high anisotropy of resistance is considerably moresatisfied by brushes made of natural graphite. With these, the ratio oftransverse resistivity to longitudinal resistivity is 4:1 to 6:1.Brushes of natural graphite however, have the disadvantage ofwithstanding relative low current loads only. Modern motors, for examplerailroad motors, operating with high current intensities amounting onthe average to 10-12 amp/cm. and with "peak loads of 2022 amp/cmfi, thuscall for the use of electrographite brushes because of their much highercurrent-carrying capacity.

It has time and again been attempted to produce electrographite brusheswith a higher ratio of transverse to longitudinal resistivity. For thispurpose, for example, so-called layer-type brushes or sandwich brusheshave been produced. These are composed of several individual layersextending transverse to the longitudinal direction and being cementedtogether. By employing an insulating cement, the longitudinalresistivity of such brushes can be kept unchanged while considerablyincreasing the transverse resistivity. The drawback of this type ofbrushes, however, resides in the fact that the cement masses are notable to withstand the high mechanical, electrical and thermal stressesoccurring in the running surface of the brush. At this surface thereoccur high temperatures partly due to arcs and partly due to transienthigh contact resistances, and these high temperatures overstress theinsulating mass composed essentially of different kinds of syntheticresinous plastics. As a result, the brushes lack sufficient mechanicalsta- United States Patent 0 bility in continuous operation and sufferfrom reduced insulation ability at the running surface due to formationof conducting coke bridges. Such impairments greatly reduce theinitially high transverse resistance of the brush. Such brushes,therefore, still leave much to be desired.

It is further known to subdivide brushes twice or three times, using atwin or triplet brush instead of a single homogeneous block-type brush.It has been found, however, that in practice such brushes are subjectedto nonuniformities with respect to current distribution.

Recently there have been developed methods which afford the productionof carbon fibers by carbonization of natural fibers and which alsopermit the production of mesh material or woven fabrics of carbon bycarbonization of natural-fiber fabrics (US. Patent 3,297,405). Suchcarbon fibers or webs can be graphitized and thus be converted tographite fibers or graphite woven webs. Fibers and webs of this kindhave very high electrical resistivities. Their electrical resistance isgreatly dependent upon orientation; the resistance in the fiberdirection being considerably lower than transversely thereof. It is alsoknown to compress several layers of the graphite web while applying acarbonizable binding agent, and to convert the resulting body to coke orsubsequently to graphite. The artificial bodies of carbon or graphiteare applicable for a variety of purposes, including commutator brushes.The current-carrying capacity of such brushes, however, is lower thanthat of the conventional electrographite brushes.

It is an object of my invention to devise a commutator brush having ahigh ratio of transverse to longitudinal resistance but avoiding theabove-mentioned disadvantages of the brushes heretofore available.

According to the invention, a commutator brush, generally of theabove-described type, is provided with one or more layers of meshmaterial formed of artificial carbon which is embedded within the bulkmaterial of which the body of the brush is otherwise constituted, themesh layer extending longitudinally of the brush body and preferablyconsisting of a web or woven fabric made from animal fiber such as wool.

A brush structure thus composed is illustrated by Way of example on theaccompanying drawing which schematically shows a longitudinal crosssection.

The body 1 of the brush consists essentially of industrial carbon whichneed not be different from that employed for conventional brushes.Embedded Within the body of carbon and integrally bonded therewith aretwo woven web layers 2 and 3 consisting of artificial carbon orgraphite. The two layers are parallel ,to each other. The spacingbetween them is approximately equal to the spacing of each from thenearest longitudinal side of the brush body, although it will beunderstood that other distance relations are applicable and that alarger or smaller number of embedded mesh layers may be provided.

A brush as exemplified by the above-described embodiment is made bypressing the green mass of bulk material in a die or other mold in theconventional manner but placing the web layers of artificial carbon intothe green mass so that upon completion of the pressing operation theinserted web material is either located in the middle of the pressingheight of, as shown on the drawing, as approximately the positionindicated. In this manner, the brush body being molded and pressed maybe provided with two or more inserted webs of woven carbonaceousmaterial.

The pressing operation forces the particles of the green mass into themesh openings of the web of artificial carbon. Thus the green massenters into an intimate and fast bond with the web. Since the individualthreads of carbon or graphite have a porous structure, a furtheranchoring in the brush body is achieved by virtue of the fact thatbinding agents, contained in the green mass, will penetrate into thethreads of the web material and become bonded thereto. For thesereasons, the subsequent coking as well as any subsequently appliedgraphitization, re sults in a uniform body with a homogeneous bonding ofthe embedded webs. The bonding can be further improved by moistening orimpregnating the webs with binding agent prior to embedding the webs inthe green mass. The binding agent, for example, may be applied as aspray of tar, synthetic resin or the like plastic. Moistening of thecarbon or graphite webs with furfuryl alcohol prior to embedment isparticularly recommended.

After coking (carbonizing) the brush bodies with the embedded web layersand, if desired, after subsequent graphitizing treatment, the resultingcommutator brushes possess the desired anisotropy of the specificelectrical resistivity. The attainable ratio of transverse tolongitudinal resisivity is about 6:1 or more.

Brushes according to the invention have the further advantage ofexcellent running qualities. This is due to the fact that the carbon orgraphite fibers of the embedded web are of the same kind as the bulkmaterial of the .brush body, and for that reason are no cause oftrouble.

It has been found preferable to employ embedded webs' of fabricsproduced by carbonization of animal fibers, for example wool.

The embedding of a web of artificial carbon into the brush body may alsobe effected by inserting into the green mass at web that is not yetcompletely coked (carbonized) especially a web of animal fiber material,whereafter the conventional carbonizing and, if desired, graphitizingoperation is performed. When thus employing an incompletely carbonizedweb layer, the subsequent carbonization of the shaped brush body causescracking, and the evolving reaction products improve the bonding betweenthe webs and the bulk material.

The invention will be further described with reference to examples.

EXAMPLE 1 The green mixture is prepared from lamp black (noodle) cokepowder and tar-pitch binder. When ready for pressing, the mixture isfilled into a mold of 100 mm. by 150 mm. cross section, up to a heightof 30 mm., and the top is smoothed to planar shape. Then a roughly wovenmesh of artificial carbon, of 0.7 mm. thickness is placed on top andcovered with the same quantity of green mixture. Thereafter, a pressureof 2 metric tons per cm. is applied for molding the laminated body.

The pressed bodies are inserted into crucibles and fired in a gas-heatedchamber furnace at a temperature up to 900 C. Thereafter, a graphitizingtreatment is applied by subjecting the fired body to a maximumtemperature of 2800 C. The resistivity in the pressing direction, i.e.perpendicular to the plane of the embedded web, was measured as 290 ohmmrnF/m. The resistivity parallel to the web was 50 ohm mm. /m. Thiscorresponds to an anisotropy coefiicient of electrical resistivity inthe amount of 5.8.

EXAMPLE 2 The green mixture of Example 1 is pressed in a mold asdescribed, except that two layers of artificial carbon fiber areemployed in the form of webs made of fine threads and having a thicknessof 0.2 mm. For this purpose, one third of the total quantity of greenmixture is placed into the press mold and then covered with the firstweb layer. Thereafter, the second third of the green mixture quantity isplaced into the mold. After the second web is put on top, the remainingthird of the mixture quantity is filled in. To improve bonding, theartificialcarbon webs are sprayed with furfuryl alcohol prior to placingthem into the mold. The further fabrication is in accordance wtihExample 1. The electrical resistivity of the graphitized brushes wasmeasured as:

Ohm mmP/m. In the direction of the webs 42 Perpendicular to the webs 255This coresponds to a 6.1 ratio of transverse to longi tudinalresistivity.

EXAMPLE 3 In the web direction-19 ohm mm. /m.

.Perpendicularly to the webs-97 ohm mm. /m.

Anisotropy-5.1

I claim:

1. A commutator brush comprising carbon bulk material and at least onelayer of artificial carbon characterized by a transverse-to-longitudinalresistance ratio of at least 5.1/1.00'.

2. In a commutator brush according to claim 1, said layer consisting ofa woven web of carbonized animal fiber.

References Cited UNITED STATES PATENTS 531,707 1/1895 Wallace 310-248701,369 6/1902 Mills 310-248 1,117,965 11/1914 Becker 310-248 1,556,99010/ 1925 Henry 310-248 2,33 8,409 1/1944 Conradty 310-248 3,297,405 1/1967 Sperk 23-2091 FOREIGN PATENTS 17,688 8/ 1902 Great Britain.

ORIS L. RADER, Primary Examiner L. L. HEWITT, Assistant Examiner U.S.Cl. X.R. 310-252

